The recent increase in small electronically powered devices has increased the demand for alkaline electrochemical cells. The most widely used types of alkaline cells are zinc air cells and zinc metal oxide cells.
Zinc air cells, or "button cells", are electrochemical cells wherein the oxygen in the air acts as the cathode material and amalgamated zinc is the anode material. Air enters the cell through ports in the cell which are immediately adjacent to a cathode assembly. The air diffuses into an air cathode subassembly where the oxygen is reacted. This air cathode subassembly generally consists of mixtures of activating chemicals supported by a complex physical structure. The cathode is in chemical contact with the cathode such that oxygen can diffuse into the electrolyte through a moisture barrier usually of a plasticlike material which is impervious to liquids (including the alkaline electrolyte). The oxygen then reacts with the water in the electrolyte consuming electrons and producing hydroxide ions. These ions then migrate through an electrical barrier which is transparent to liquid ions where they oxidize the metallic zinc, which is also in chemical contact with the electrolyte, generally producing two electrons for each atom of zinc reacted. Such cells are well known and are more fully discussed in references such as U.S. Pat. Nos. 3,149,900 (Elmore and Tanner), 3,276,909 (Moos) and 4,617,242 (Dopp).
Zinc air button cells are normally constructed in two steps. The anode section and the cathode section are usually separately assembled and the joined together prior to the cell being permanently sealed. Generally, the cathode section is contained in a topless, hollow metallic can with a small air entry hole in the can bottom. The air cathode subassembly, which is covered with a nonmetallic separator, is slightly compressed within the cathode section. The zinc anode section consists of a topless, hollow metallic can into which a measured amount of zinc is placed. The alkaline electrolyte is then metered directly onto the surface of the zinc. After the electrode sections have been made, they are joined by inverting the cathode section and placing it into the open end of the anode can. The button cell is then sealed, usually by crimping the edges into a nonmetallic grommet.
Zinc-metal oxide cells also employ zinc as an anode material. However, the cathode is a metal oxide, such as manganese dioxide or silver oxide, which is either compressed into a compacted pellet or finely dispersed in a gelatinous material. Zinc-metal oxide button cells are constructed much as the zinc air button cells described above. However, the air cathode is replaced by the metal oxide pellet or gel which is separate from the anode when the cell is assembled by a separator containing electrolyte.
Zinc-metal oxide cells are more commonly found in the form of cylindrical batteries. Cylindrical batteries are made by enclosing the metal oxide gel and electrolyte in a cylindrical "zinc can" which acts as the anode. The cathode gel and electrolyte are introduced into the zinc can such that the gel and electrolyte form separate layers, with the electrolyte layer separating the cathode gel from the zinc can such that both the cathode and anode are in chemical contact with the electrolyte. The can, electrolyte and/or cathode gel can be separated by barriers which are permeable to ions. A current collector, such as a carbon rod, is then inserted into the cathode gel. Terminals are then contacted with the can and the collector and the battery is encased in an insulating sleeve and/or metal, paper or polymer jacket.
Additional information relating to alkaline cells and their construction can be found in Vincent, "Modern Batteries: An Introduction to Electrochemical Power Sources" (Edward Arnold 1984), which is incorporated herein by reference.
In manufacturing alkaline cells, care must be taken during the assembly process to allow no paths for leakage of electrolyte from the cell, thus maintaining the integrity of cell function and structure and keeping the caustic electrolyte contained. Unfortunately, the mechanized processes for manufacturing these cells on occasion cause electrolyte to spill or escape from the cell. This is most likely to occur immediately prior to crimping the cell components tightly together and while the cell construction is inside the closing die. If leaks and spills are not prevented and surfaces of the cell become wetted by electrolyte, electrolyte will continue to leak from the cell regardless of the sealing technique used in the production process. Escaping electrolyte may also cause cells to stick to or in automatic equipment used in the manufacturing process (e.g., to the closing die) leading to equipment failure and manufacturing delays. The present invention provides reliable means for detecting the presence of even a very small quantity of electrolyte outside of a cell both before and after closure of the cell by providing a fluorescent electrolyte which is visible upon exposure to radiation.
Generally, it is also desirable to increase the capacity of cells whenever possible. It is, therefore, an additional object of the present invention to provide means for improving cell capacity.